1. Field of the Invention
This invention generally relates to computer-implemented methods, computer-readable media, and systems for identifying one or more optical modes of an inspection system as candidates for use in inspection of a layer of a wafer. Certain embodiments relate to a computer-implemented method for eliminating a first portion of different optical modes available on an inspection system as not candidates for use in inspection based on one or more characteristics of images acquired using the different optical modes.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Fabricating semiconductor devices such as logic and memory devices typically includes processing a substrate such as a semiconductor wafer using a large number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that involves transferring a pattern from a reticle to a resist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a single semiconductor wafer and then separated into individual semiconductor devices.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on a specimen such as a reticle and a wafer. Inspection processes have always been an important part of fabricating semiconductor devices such as integrated circuits. However, as the dimensions of semiconductor devices decrease, inspection processes become even more important to the successful manufacture of acceptable semiconductor devices. For instance, as the dimensions of semiconductor devices decrease, detection of defects of decreasing size has become necessary since even relatively small defects may cause unwanted aberrations in the semiconductor devices. Accordingly, much work in the inspection field has been devoted to designing inspection systems that can detect defects having sizes that were previously negligible.
Inspection for many different types of defects has also become more important recently. For instance, in order to use inspection results to monitor and correct semiconductor fabrication processes, it is often necessary to know what types of defects are present on a wafer. In addition, since controlling every process involved in semiconductor manufacturing is desirable to attain the highest yield possible, it is desirable to have the capability to detect the different types of defects that may result from many different semiconductor processes. The different types of defects that are to be detected may vary dramatically in their characteristics. For example, defects that may be desirable to detect during a semiconductor manufacturing process may include thickness variations, particulate defects, scratches, pattern defects such as missing pattern features or incorrectly sized pattern features, and many others having such disparate characteristics.
Many different types of inspection systems have been developed to detect the different types of defects described above. In addition, most inspection systems are configured to detect multiple different types of defects. In some instances, a system that is configured to detect different types of defects may have adjustable image acquisition and sensitivity parameters such that different parameters can be used to detect different defects or avoid sources of unwanted (nuisance) events. For instance, the spot size, pixel size, or polarization or algorithm settings for angles of collection may be different for an inspection process used to detect particulate defects than for an inspection process used to detect scratches.
Although an inspection system that has adjustable image acquisition and sensitivity parameters presents significant advantages to a semiconductor device manufacturer, these inspection systems are useless if the incorrect image acquisition and sensitivity parameters are used for an inspection process. For example, incorrect or non-optimized image acquisition and sensitivity parameters may produce such high levels of noise that no defects can be detected in the generated inspection data. In addition, since the defects, process conditions, and noise on wafers may vary dramatically (and since the characteristics of the wafers themselves may vary dramatically), the best image acquisition and sensitivity parameters for detecting the defects on a particular wafer may be difficult, if not impossible, to predict. Therefore, although using the correct image acquisition and sensitivity parameters will have a dramatic effect on the results of inspection, it is conceivable that many inspection processes are currently being performed with incorrect or non-optimized image acquisition and sensitivity parameters.
The task of setting up an inspection process for a particular wafer and a particular defect of interest (DOI) may be extremely difficult for a user, particularly when an inspection system has a relatively large number of adjustable image acquisition settings and sensitivity parameters. In addition, it may be impossible to know whether the best inspection process has been found unless all possible combinations of the image acquisition parameters have been tested. However, most inspection processes are currently set up using a large number of manual processes (e.g., manually setting the image acquisition parameters, manually analyzing the resulting inspection data, etc.). As such, setting up the inspection process may take a relatively long time. Furthermore, depending on the types of wafers that will be inspected with the inspection system, a different inspection process may need to be set up for each different type of wafer. Obviously, therefore, setting up the inspection processes for all of the different wafers that are to be inspected may take a prohibitively long time.
Some previously used methods for selecting an optimum optical mode for inspection of a layer of a wafer include inspecting a wafer using a default mode and aggressive detection settings, generating a review sample, and reviewing the wafer using the output of the inspection system (often in the form of a KLA-Tencor review file also commonly referred to as a KLARF) on a scanning electron microscope (SEM) in search of DOI and nuisance events. Such methods may also include classifying the reviewed defects, collecting images of each defect and a reference image using a number of modes where the set of defects for which images are grabbed is a subset of the review sample based on a distribution of class codes. In addition, such methods may include performing signal-to-noise (S/N) analysis on the images collected, inspecting the wafer again at the mode or modes with the most favorable S/N values, generating a new review sample, performing additional SEM review, and optimizing inspection threshold parameters until nuisance defect detection rates are acceptable.
Such methods, however, have a number of disadvantages. For example, in such methods, identifying DOI is required in order to evaluate a mode. However, it is possible and likely that the default mode will not detect the DOI. Discovery that DOI was not detected happens well into the process at the SEM review step. When this occurs, the user must go back to the start of the process and choose another mode with which to scan. Failing to detect DOI using the default mode, therefore, results in wasted time and can be substantially severe if the DOI is continually not detected by the mode(s) used for scanning the wafer.
Another disadvantage is that even if DOI are detected at the first step of the process, the collection of images for a reasonable sample of DOI and nuisance events at every inspection mode is impossible due to time constraints. Without images at a particular mode, the methods described above are not able to evaluate that mode. Since unevaluated modes are no longer candidates for mode selection, there is a risk that valuable modes have been discarded. Even when only a subset of modes are evaluated, the time required is substantially long.
An additional disadvantage is that the methods described above rely on the existence and identification of DOI and the evaluation of each mode using these DOI. However, there are cases in which a user would like to choose an optimal mode or modes using a setup wafer that does not have DOI or without identifying DOI on a wafer regardless of whether DOI are located thereon. The only recourse in the methods described above is to use the default mode which has a relatively high risk of not being an optimal mode.
Accordingly, it would be advantageous to develop computer-implemented methods, computer-readable media, and/or systems for identifying one or more optical modes of an inspection system as candidates for use in inspection of a layer of a wafer that are less labor intensive, quicker, and less tedious than previously used methods and that result in optical modes selected for inspection of a layer on a wafer that are more appropriate, or even optimal, for the inspection than optical modes selected using the methods described above.